The invention is related to the field of optics, and in particular to a technique of forming a periodic, three-dimensionally micro-structured material with a two-dimensional variation in micro-structure duty cycle.
Coherent diffraction lithography, also known as the Talbot effect and the self-imaging effect, is a method for optically replicating periodic structures. The effect occurs when a periodic amplitude or phase mask is illuminated by coherent light. Optical images of the periodic mask are formed at interval distances behind the mask, known as Talbot distances. By placing a photo-film at one Talbot distance, the grating is copied by exposing and developing the film. If the photopolymer is several Talbot distances thick, the periodic pattern will be copied into it at each Talbot distance. Exposing and developing the film will create a periodic porous micro-structure out of the polymer. The 2D periodic pattern of the exposure mask determines what 3D geometry is formed. Many 3D geometries have been made including face centered cubic and body centered tetragonal. Varying the exposure dose controls what fraction of the photopolymer remains behind, or how thick the micro-structure is.
These 3D structures are typically made in photopolymers such as SU-8 but could be made in any photosensitive material using either one-photon or two-photon exposure. The main limitation of the Talbot effect is that it can only be used to replicate periodic patterns. This is very well known and efforts to replicate non-periodic patterns, even for periodic patterns with single element defects, do not work. This is because the patterns become blurred out in the photopolymer.